Over the past 50 years many versions of VTOL aircraft have evolved. VTOL aircraft may be conveniently classified in accordance with their method of converting vertical flight to horizontal flight. The first method is to provide a rotor or similar means rotating about a vertical axis for vertical flight and the entire aircraft is tilted for horizontal flight. A conventional helicopter fits into this category. After travelling vertically to a safe distance off the ground by the lift of its rotor, the pilot causes the aircraft to tilt in the direction he wants to go.
A second method is the tail-setting approach using craft nicknamed "pogos" because they are provided with means to allow them to be positioned on their tails and take off and land vertically. Phases of flight other than take offs and landings are flown with the longitudinal axis of the aircraft maintained in a substantially normal horizontal orientation. This method was made feasible by the advent of propulsion units that could produce an installed thrust to weight ratio greater than one.
Tilting only the rotors, propellers, or other sources of thrust is the third method of achieving VTOL operation. In some designs, the lifting surfaces provided also tilt along with the source of thrust, but in all aircraft falling into this category, the fuselage remains in its normal substantially horizontal orientation during takeoff and landing.
The fourth method is to maintain the aircraft in its normally horizontal flight attitude but to deflect or vector thrust. High-velocity air forced back by propellers or the efflux from jet engines is turned downwards by means such as large flaps (deflected) or by means of nozzles (vectored).
Dual propulsion is the fifth method of achieving VTOL operation. In this category, the aircraft has separate engines dedicated to lifting and lowering the aircraft and separate engines for horizontal flight. Aircraft of this type are also called direct lift aircraft.
It will be recognized that the five preceding methods for achieving VTOL operation have one thing in common: the propulsion efflux is directed downward to produce lift by a consequent reaction to the downwash momentum. However, it is also known in the prior art that lift can be produced by yet another means which may be conveniently termed "upper surface blowing". Upper surface blowing comprises the acceleration of the flow over the upper surface of a body to create a drop in pressure thereover relative to the pressure on the under surface of the body, producing lift forces on that body. Upper surface blowing produces lift without a requirement for a turning downward of the flow that has passed over the body being lifted.